Overview of Sea Lettuce Biology

Ulva lactuca, or Sea Lettuce, is a green macroalgae in the Ulvaceae. It has a worldwide distribution, thriving in both temperate and tropical coastal regions. It often is associated with high-nutrient, intertidal zones where it provides food to herbivores, habitat to inverts and oxygen to the water column .

Sea Lettuce can be light to dark green in appearance and grows in a doubled-layered sheet. This “lettuce leaf” can grow in suspension, but will eventually form a disc-shaped holdfast to anchor itself to substrates. Ulva lactuca has a complex sexual life cycle, but adult plants can be readily reproduced by cuttings. Sea Lettuce is a charismatic example of green algae and an aggressive consumer of inorganic nitrates and phosphates. This photosynthetic capability allows Ulva to convert these waste products into valuable nutritious compounds. This nutrition reenters the ecology when herbivores graze on vast fields of sea lettuce.

The Nutrition of Sea Lettuce

Ulva’s greatest promise to the reef aquarium industry is to be a nutritious, sustainable feed to a wide array of herbivorous and omnivorous species. Ulva is able to produce and retain some golden fats, but produces considerable amounts of protein, vitamins and minerals. When the Ulva is fed off, this nutrition is presented in a bio-active form, complete with probiotics, prebiotics¹ and proenzymes². All of these are essential to establishing and maintaining the stable gut microbiome essential for the long-term health of herbivorous reef fish such as Acanthurans (tangs) and Siganids (rabbitfish).

Because of its nutritional profile, Ulva has been incorporated into the feeds of broiler chickens, goats, cows, african catfish, pacific white shrimp, rabbitfish, milkfish, barramundi, gilthead sea bream, white-spotted snapper, sea urchins, abalone and sea cucumbers to beneficial effect. In a few species, Ulva meal could be used to 100% replace fish meal. This makes it a powerful and sustainable food item for the reef.

Converting Waste to Nutritious Sea Lettuce

Wild Ulva thrives both in nutrient rich estuaries and in the shadow of reefs. In India and the Philippines, Ulva is grown at scale to bioremediate fish/shrimp ponds. This is because it can aggressively utilize a variety of nutrient sources. Like Chaetomorpha, Ulva prefers to utilize ammonia for photosynthesis but will switch to nitrate and phosphate once ammonia is depleted. This means that a robust Ulva garden refugium coupled with a highly-functional biofilter can suppress fluxes in nitrates and phosphates. Though it will survive at lower light levels, Ulva is most powerful when exposed to prolonged periods of intense lighting. As a green algae, it particularly enjoys red spectrum light.

 

*To maximize nitrate/phosphate absorbing power of Ulva, provide it with 250+PAR light with as much red spectrum as possible.

 

When housed under a properly-lighted refugium, Ulva converts toxic nitrogenous wastes into nutritious Sea Lettuce which can then be fed off to the display tank. In this way, an Ulva-garden has dual function as a biofilter and as a continual source of high-quality tang chow!

Heavy-Metal Sink

Ibrahim et al 2016 demonstrated Ulva’s tremendous ability to bioabsorb heavy metals ions such as copper, chromium, cadmium and lead from industrial wastewater. This means robust Ulva gardens may have a role in regulation of trace metals in the reef aquaria as well. This may be especially helpful for aquarists which do not have reliable access to RO water.

Oxygenates the Refugium & Provides Substrate for Pods

Another hallmark of Ulva’s aggressive photosynthesis is that it can supersaturate the water around its tissues with oxygen. This, combined with its huge waving blades, makes Ulva an EXTREMELY attractive home for copepods and other refugium inverts. Because it grows in these sheetlike blades, it is important to periodically feed off excess Ulva in the refugium to ensure that it does not smother itself...but we are sure your herbivores will appreciate every harvest!

*Stock Ulva when initially seeding pods to assist in the colonization of your reef.

Suppresses Pest Microbes & Enriches Reefs

Because it is such an aggressive photosynthesizer, Ulva suppresses the growth of pest algae by depriving them of excess nutrients. It has also evolved to directly attack the microbes around it. Egan et al 2000 demonstrated how the presence of Ulva inhibited growth of fungi and bacteria and prevented algae spores from germinating. Furthermore, there is mounting evidence that benthic algae may release bioactive compounds which benefit coral reef ecosystems.

“Recent research indicates that coral reef associated benthic algae may control important metabolic processes in reef ecosystems via organic matter release. Yet little information is available about quantity and chemical composition of these algae-derived exudates.”³

Conclusion

Sea Lettuce is a valuable addition to any refugium because it is functional in every way. It compliments the biofilter in eradication of nitrogenous wastes while providing an oxygenated home for pods. Its greatest benefit is that it is able to rapidly convert inorganic waste into highly palatable, highly nutritious reef veggies!

A Note on Buying Clean Macroalgae

 The reason I always recommend the purchase of fresh macroalgae culture when starting a new tank, is the security of knowing that your seaweed is free of pests and hitchhikers. Pod Your Reef guarantees that all of its Ulva is 100% quarantined, treated and pest-free.

 

¹ Food for probiotics

² Building-blocks of enzymes 

³ Haas, A. F., Naumann, M. S., Struck, U., Mayr, C., el-Zibdah, M., & Wild, C. (2010). Organic matter release by coral reef associated benthic algae in the Northern Red Sea. Journal of Experimental Marine Biology and Ecology, 389(1-2), 53-60.

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